Nanoscale sensors, in various forms, have been under intense research focus in the past several years due to their high sensitivity toward changes in many physical parameters such as mass, force, energy, stress, temperature, charge, and spin. Additionally, these systems promise high integration density and low power consumption, two of the most desirable aspects of an integrated system. A combination of all these properties can lead to potential applications of these sensors in a large variety of civilian and military applications. There are currently two approaches for nanoscale sensor fabrication: (1) top-down and (2) bottom-up. In the top-down approach, expensive and complicated fabrication processes are utilized to realize the nanostructures, which limits their applications to only very specific and niche areas. In the bottom-up approach, the nanostructures are realized through inexpensive nanowire (NW) synthesis processes that can open up opportunities for widespread applications. In addition, the uniformity and quality of the nanostructures (which seriously impact the device characteristics) that are naturally obtained during the synthesis process cannot be achieved by current state-of-the-art nanofabrication tools. However, controlled positioning of the nanowires over a large area has been quite difficult to achieve so far, restricting their applications to individual devices, or to low levels of integration.
Nanoelectromechanical systems (NEMS) constitute a very important branch of nanoscale sensors where the system stimulation and transduction is performed by electrical means, but the actual sensing is performed mechanically, taking advantage of the exceptionally high quality factors (in resonance) available in these systems that are normally not possible to achieve in electrical systems. There have been attempts to fabricate NEMS devices based on nanowires and nanotubes (NTs). However, in addition to the problem of integration as mentioned above, NEMS sensors in general, and those based on nanowires (or nanotubes) in particular, suffer from the problems of transduction of the mechanical signal into electrical form. Various techniques have been employed to transduce the mechanical deflection of the NEMS device, which includes optical, electron beam, magnetic, radio-frequency transmittance, and piezoresistive. However, with the exception of piezoresistive transduction, none of these techniques are applicable for simultaneous deflection transduction of multiple NEMS devices in an integrated circuit. Another significant limitation with most NEMS sensors reported in the literature is they almost invariably have a linear geometry (due to obvious ease in fabrication and alignment), and are fixed at both ends, acting as a beam resonator rather than a cantilever resonator. This geometry greatly reduces the sensitivity of these devices to changes in physical quantities that needs to be measured.
Other major characteristics of nanoscale sensors are strongly influenced by the properties of the material (usually semiconductor) from which it is made. Usually, for chem-FET type sensing (based on surface depletion caused by adsorbed molecules) the material should have a high carrier density, high mobility, chemical inertness, and thermal stability. NEMS devices would additionally require smooth and highly crystalline nanowires for high quality factor, and large piezoresistivity for efficient transduction of device deflection. Most semiconductors and their nanowires display some of the above mentioned properties, but rarely all.
Thus, a need exists for an improved geometry and material construction of the semiconductor material for the cantilever used as the sensor to improve detection sensitivity and signal transduction.